Glazings consisting of compositions containing transparent thermoplastic polymers such as polycarbonate, for example, offer many advantages for the vehicle domain and for buildings, compared to conventional glazings consisting of glass. These advantages include, for example, increased fracture resistance or saving of weight, which in the case of automobile glazings enable higher occupant safety in the event of traffic accidents, and a lower fuel consumption. Lastly, transparent materials that contain transparent thermoplastic polymers permit a significantly greater design freedom by reason of their simpler mouldability.
A disadvantageous aspect, however, is that the high diathermancy (i.e. permeability in respect of IR radiation) of transparent thermoplastic polymers leads, in the case of solar influence, to an undesirable heating in the interior of vehicles and buildings. The increased temperatures in the interior reduce the comfort for the occupants or inhabitants and may entail increased demands on the air conditioning, which in turn intensify the energy consumption and in this way cancel out the positive effects. In order nonetheless to take into account the demand for low energy consumption combined with a high degree of comfort for the occupant, windowpanes are required that provide an appropriate thermal-protection. This applies, in particular, to the automobile domain.
As generally known the greatest part of the solar energy is apportioned both to the visible region of light between 400 nm and 750 nm and to the region of the near infrared (NIR) between 750 nm and 2500 nm. Penetrating solar radiation is, for example, absorbed in the interior of an automobile and emitted as long-wave thermal radiation with a wavelength from 5 μm to 15 μm. Since in this region customary glazing materials—in particular, thermoplastic polymers that are transparent in the visible region—are not transparent, the thermal radiation cannot radiate outwards. A greenhouse effect is obtained, and the interior space heats up. In order to keep this effect as small as possible, the transmission of the glazings in the NIR should therefore be minimised as far as possible. However, customary transparent thermoplastic polymers—such as polycarbonates, for example—are transparent both in the visible region and in the NIR.
Therefore admixtures, for example, are needed that exhibit a transparency in the NIR that is as low as possible without disadvantageously influencing the transparency in the visible region of the spectrum.
Amongst the transparent thermoplastic synthetic substances, polymers based on polymethyl methacrylate (PMMA) and polycarbonate are particularly well suited for use as glazing material. By reason of its high toughness, polycarbonate in particular possesses a very good property profile for end uses of such a type.
In order to impart heat-absorbing properties to these synthetic substances, appropriate infrared absorbers are therefore employed as additives. Particularly of interest for this purpose are IR-absorber systems that are provided with a broad absorption spectrum in the NIR region (near infrared, 750 nm-2500 nm) with, at the same time, low absorption in the visible region (slight intrinsic colour). Furthermore, the corresponding polymer compositions exhibit a high thermostability and also an excellent light stability.
A large number of IR absorbers based on organic or inorganic materials are known that can be employed in transparent thermoplastics. A selection of materials of such a type is described, for example, in J. Fabian, H. Nakazumi, H. Matsuoka, Chem. Rev. 92, 1197 (1992), in U.S. Pat. No. 5,712,332 or JP-A 06240146.
However, IR-absorbing additives based on organic materials frequently have the disadvantage that they exhibit slight stability in relation to thermal loading or irradiation. Accordingly, many of these additives are not sufficiently thermally stable to be worked into transparent thermoplastics, since temperatures up to 330° C. are required in the course of their processing. Furthermore, in use the glazings are often exposed over lengthy periods to temperatures of more than 50° C., caused by the solar radiation, which may result in the decomposition or degradation of the organic absorbents.
Furthermore, the organic IR absorbers frequently do not exhibit a sufficiently broad absorption band in the NIR region, so that their use as IR absorbers in glazing materials is inefficient. Moreover, a strong intrinsic colour of these systems often also arises, which as a rule is undesirable.
In comparison with organic additives, IR-absorbing additives based on inorganic materials are frequently distinctly more stable. The use of these systems is often also more economical, since in most cases they exhibit a distinctly more favourable cost/performance ratio. Accordingly, materials based on fine-particle borides, such as lanthanum hexaboride for example, have proved to be efficient IR absorbers, since they are provided with a broad absorption band combined with a high thermostability. Such borides based on La, Ce, Pr, Nd, Tb, Dy, Ho, Y, Sm, Eu, Er, Tm, Yb, Lu, Sr, Ti, Zr, Hf, V, Ta, Cr, Mo, W and Ca are described, for example, in DE 103 92 543 T5 or EP 1 559 743 A1.
A disadvantage of these additives, however, is their significant intrinsic colour. After being worked in, the boride-containing additives impart a characteristic green colouration to the transparent synthetic substance, which is frequently undesirable, since it greatly restricts the scope for a neutral colouring.
For the purpose of compensating for the intrinsic colour, often relative large quantities of further colouring agents are employed, which, however, impairs the optical properties of the composition and results in a distinctly diminished transmission in the visible region. Particularly in the case of vehicle glazings this is undesirable or—in special cases in which the view of the driver must not be impaired-impermissible.
Furthermore, IR-absorbing additives from the group of the tungsten compounds are known that provide a lower self-absorption in the visible spectral region in comparison with the inorganic IR absorbers based on boride that are known from the state of the art.
The production and use of these substances in thermoplastic materials are described, for example; in H. Takeda, K. Adachi, J. Am. Ceram. Soc. 90, 4059-4061, (2007), WO 2005/037932 A1, JP 2006 219662 A, JP 2008 024902 A, JP 2008 150548 A, WO 2009/059901 A2 and JP 2008 214596 A. However, the deficient long-term stability in relation to thermal loading turned out to be disadvantageous. Whereas the thermal instability of tungsten oxides is known as such and has been described, for example, in Romanyuk et al.; J. Phys. Chem. C 2008, 112, 11090-11092, it became evident also in the case where these compounds are worked into a polymer matrix that in the course of thermal storage at elevated temperature of the corresponding polymer compositions—such as, for example, in the case of a polycarbonate composition—the absorption in the IR region declines significantly.
For an application of the compositions in the glazing field, in particular for car glazings, it is, however, absolutely essential that the corresponding IR-absorbing polymer compositions exhibit a long-term stability in relation to higher temperatures. By the term ‘higher temperatures’, temperatures are meant, for example, that an article consisting of polycarbonate may assume in the case of intense solar radiation (for example, 50° C.-110° C.). Furthermore, it has to be guaranteed that the compositions can be processed under conventional process conditions without the IR-absorbing properties being diminished as a result.
Furthermore, for the purpose of improving the processing properties in thermoplastic materials it was known to use thermostabilisers such as, for example, phosphites, hindered phenols, aromatic, aliphatic or aliphatic/aromatic phosphines, lactones, thioethers and hindered amines (HALS, hindered amine light stabilizers).
From WO-A 01/18101 moulding compounds containing a thermoplastic synthetic substance and a phthalocyanine dye or naphthalocyanine dye are known which for the purpose of improving the processing stability may contain antioxidants such as phosphites, hindered phenols, aromatic, aliphatic or mixed phosphines, lactones, thioethers and hindered amines. In contrast, the present invention relates to compositions containing inorganic IR absorbers based on tungsten.
From EP 1 266 931 A1 organic IR absorbers in polycarbonate compositions are known in combination with phosphines. However, no reference to the combination of inorganic IR absorbers—in particular, inorganic IR absorbers based on tungsten—with phosphines for the purpose of stabilising the absorbers in a thermoplastic matrix is described in EP 1 266 931 A1.
In EP 1 559 743 A1 polycarbonate compositions are described containing inorganic IR absorbers based on borides in combination with thermostabilisers such as phosphonites and phosphines, these additives serving for stabilising the polycarbonate matrix. Tungsten-based compositions are not described. It is not known that the aforementioned stabilisers have an influence on inorganic IR absorbers.
US 2006/0251996 A1 discloses multi-layer sheets containing a core layer containing a thermoplastic polymer and an IR-absorbing additive, the IR-absorbing additive being a metal oxide. Furthermore, the core layer may additionally contain thermostabilisers. A polymer composition with an IR absorber stabilised by phosphine according to the present invention and also master batches stabilised with phosphines are, however, not described in US 2006/0251996 A1. In particular, US 2006/0251996 A1 also does not describe the use of a nanoscale IR absorber embedded in a dispersing agent.
But in all the thermoplastic compositions with IR absorbers that have been published hitherto, the thermostabiliser serves exclusively for stabilising the respective polymer matrix—particularly in the course of processing. Accordingly, through the use of these systems the yellow colouration of the polycarbonate after exposure to light, as described in EP 1 266 931 A1, can be limited.
The object was therefore to find IR-absorbing systems with low intrinsic colour and also with, at the same time, high thermostability and stability in relation to exposure to light, and to make available corresponding compositions with thermoplastic materials. At the same time, these additives are to be provided with a broad absorption characteristic in the NIR region, whilst they exhibit an economically justifiable or even interesting cost/performance ratio. A further object of the present invention was to provide stabilisers that distinctly improve the long-term stability of known IR absorbers, and also the provision of compositions with IR absorber and stabiliser in high concentration in a thermoplastic polymer by way of master batch for further processing.
Surprisingly, it became evident that certain stabilisers improve the thermostability of IR-absorbing tungstates, in particular that of caesium tungstate, so that the object of the present invention is achieved by compositions with IR-absorbing additives from the group of the tungstates, which are provided with a lower self-absorption in the visible spectral region in comparison with the inorganic IR absorbers based on boride that are known from the state of the art, and result in thermoplastic materials with slighter intrinsic colour, in which the inorganic IR absorbers are stabilised with a stabiliser from the group of the phosphines for a higher long-term stability in relation to thermal loading.